Energy storage is a crucial component of a large number and variety of electronic devices, particularly for mobile devices and vehicles. Energy storage devices include capacitors, which store charge for later discharge through a circuit, and batteries, which rely on ions and chemical reactions to store and release energy. Existing energy storage devices have many disadvantages, including low energy storage density, slow charge/discharge performance, and high rates of degradation. For example, the low storage density in existing batteries typically requires design of large and heavy devices to store a sufficient amount of energy.
The size and weight requirements can reduce the performance of devices that a battery is powering, such as in hybrid or electric vehicles. Batteries have low storage density due to the large size and weight of the ions stored in the batteries. Transporting the ions in existing batteries also causes slow charge and discharge performance. Furthermore, the reliance of existing batteries on ionic transport causes high degradation rates of the batteries. Energy storage devices have been based on a wide variety of physical effects. For example, electric fields can be employed to store energy in capacitors, and chemical reactions (involving ion motion) can be employed to store energy in batteries. However, energy storage in a capacitor can be limited by the device geometry (e.g., 2-D capacitor plates having limited area), and batteries can have a slow response time due to the ion motion inherent in electrochemical reactions. Accordingly, it would be an advance in the art to provide energy storage having higher energy density than a capacitor, faster charge/discharge than a battery and/or much longer lifetime than a battery.